I am looking for a special tool to put a secret ingredience into a cupcake. I could use a spoon to scoop out some cake and add it. Then put the frosting on top.
The syringe is a great idea, unless your ingredients are thick. Then you'll need to go with Katie's idea - make sure the tip you choose has a thick-enough hole. (you can poke a wooden skewer into the cake first and then put the tip in.)